Extending the Use of Optical Coherence Tomography to Scattering Coatings Containing Pigments

Coating thickness is a critical quality attribute of many coated tablets. Functional coatings ensure correct drug release kinetics or protection from light, while non-functional coatings are generally applied for cosmetic reasons. Traditionally, coating thickness is assessed indirectly via offline methods, such as weight gain or diameter growth. In the past decade, several methods, including optical coherence tomography (OCT) and Raman spectroscopy, have emerged to perform in-line measurements of various subclasses of coating formulations. However, there are some obstacles. For example, when using OCT, a major challenge is scattering pigments, such as titanium dioxide and iron oxide, which make the interface between the coating and the tablet core difficult to detect. This work explores novel OCT image evaluation techniques using unsupervised machine learning to compute image metrics. Certain image metrics of highly scattering coatings are correlated with the tablet thickness, and hence indirectly with the coating thickness. The method was demonstrated using a titanium dioxide rich coating formulation. The results are expected to be applicable to other scattering coatings and will significantly broaden the applicability of OCT to at-line and in-line coating thickness measurements of a much larger class of coating formulations.

Interrelationships Between Coating Uniformity and Efficiency in Pan Coating Processes

The relationships between coating uniformity and efficiency were explored for tablet coating processes in pan coaters. The factors affecting the size of the spray zone were modeled using one-dimensional deposition analysis of spray droplets. This model was incorporated into the analytical model developed for coating uniformity by Choi et al. (AAPS PharmSciTech 22(7), 2021) that farther elucidated the effects of tablet shape and bed porosity. The results were compared with literature data on coating efficiency. The variables examined included tablet shape and size, coating time, pan speed, atomizing and pattern air flow rates, bed porosity, spray rate, batch size, coating solution concentration, spray gun-to-bed distance, and pan diameter. It is shown that, except for pan diameter and atomizing air flow rate, variables that improve coating efficiency adversely affected coating uniformity and vice versa. Implications of these relationships are discussed to improve formulation, process, and equipment designs.

Tribology provides an in vitro tool that correlated to in vivo sensory data on the mouthfeel of coated tablets

Tribology is an emerging technique in the pharmaceutical field for texture and mouthfeel studies. Due to its relevance to oral sensory perception, tribology supports the development of novel products in the food industry. This study explores tribology as a tool to optimise the mouthfeel and ease of swallowing of pharmaceutical coatings and coated tablets. We measured the lubricating properties of eight pharmaceutical coatings using two methods: surface tribology and thin film tribology. As food science is more advanced in texture and mouthfeel studies, methods were developed from this field with the intention to mimic tablet ingestion. Further, the link between tribological measurements and the sensory evaluation of the coated tablets obtained by a human panel was explored. We have demonstrated that discrimination of tablets with different coatings using tribology is feasible. The viscosity, solubility and composition of the coating formulations played an important factor in lubrication. For the first time, tribology was used to analyse the lubricating properties of conventional tablet coatings and a linear relationship between tribology and the oral sensory perception, i.e. slipperiness and stickiness, was demonstrated. Tribology has the potential to become a valuable formulation tool to characterise the lubricating behaviour of coated tablets in the context of oral sensory perception.

Sensory attributes of coated tablets: Developing a formal lexicon and sensory wheel

The patient's sensory experience when taking an oral medicine is important in the assessment of its palatability, and acceptability. The aim of this study was to develop tools useful for standardisation of sensory assessment of coated tablets: a lexicon and a sensory wheel. Two randomised, double-blind sensory assessments were performed involving 83 and 52 heathy adult volunteers and two sets of coated tablets. By adapting the principles used by food sciences, a free-text description of conventional, bitter-tasting or tasteless, coated tablets was performed. In the first assessment, volunteers described the sensory attributes of the first set of tablets. The attributes collected were then validated using a second set of tablets in a separate study with different volunteers. The appropriateness and semantics of each sensory attribute was analysed. Twenty attributes most relevant for assessment of coated tablets were selected for the lexicon and associated with explicit definitions. A collection of all attributes that could possibly be triggered by coated tablets were organised in the form of a sensory wheel. This study provides a valuable insight into the sensory experience while taking a coated tablet and presents tools which can accelerate the development of palatable medicines.

Eudragit RL-based film coatings: How to minimize sticking and adjust drug release using MAS

Polymeric film coatings based on quaternary polymethacrylates (QPMs, e.g. Eudragits®) are frequently used for controlled release applications. However, their considerable sticking tendency is a major drawback in practice. In this study, different amounts of magnesium aluminum silicate (MAS) were added to the film coatings in order to overcome this hurdle. MAS is negatively charged and can electrostatically interact with the positively charged QPM. Different types of tablet cores were coated with aqueous Eudragit® RL 30D dispersions, optionally containing varying amounts of MAS. Dynamic changes in the wet mass of the systems as well as drug release upon exposure to 0.1 M HCl and phosphate buffer pH 6.8 were monitored. Propranolol HCl, acetaminophen, and diclofenac sodium were used as cationic, nonionic and anionic model drugs. The tablets were optionally cured for 12 h at 45 or 60 °C. Importantly, the addition of MAS to aqueous Eudragit® RL 30D dispersion substantially reduced the films' stickiness and led to stable inner coating structures, even without curing. Desired drug release rates can be adjusted by varying the QPM:MAS ratio and coating level.

Determining the effect of photodegradation on film coated nifedipine tablets with terahertz based coating thickness measurements

Film coating of nifedipine tablets is commonly performed to reduce photo-degradation. The coating thickness of these tablets is a primary dictating factor of photo-stability. Terahertz spectroscopy enables accurate measurement of coating thickness. This study identifies a method to determine an end-point of a photo-protective coating process by using coating thickness measurements from terahertz time of flight spectroscopy (THz-TOF). For this method, nifedipine tablets, at different coating thicknesses, were placed in a photostability chamber. The illumination conditions of the coated tablets were adjusted based on the time duration of these tablets inside the chamber. A multiple linear regression model was developed with the coating thickness estimates from THz-TOF and illumination conditions information to predict the amount of drug remaining after photo-degradation (percent label claim). The prediction error of this model was 1.03% label claim in the range of 88.4-100.6% label claim. According to this model, acceptable levels of photo-protection in illumination conditions of up to approximately 700,000 lx hours was achieved at the end of the coating process (approximately 50 µm coating thickness) performed in this study. These results suggest THz-TOF as a viable process analytical technology tool for process understanding and end-point determination of a photo-protective coating process.

Effect of coating time on inter- and intra-tablet coating uniformity

Film tablets are a common oral dosage from. For many of the functions film layers can have on pharmaceutical tablets, a high degree of coating uniformity is required. In studies on coating uniformity the coefficient of variation is commonly used as a marker. Previous studies regarding the trend were mostly extrapolations from simulations of short coating times. Based on these it was stated that the inter tablet coefficient of variation decreases proportionally to one over the square root of coating time and hence diverge asymptotically towards zero. Extrapolations of experimental data suggested however a decrease converging to a residual value. Based on these results it can be hypothesized that the coefficient of variation decreases proportionally to one over time towards a residual value. Regarding intra-tablet coating homogeneity, no data on time dependency has been published so far. In this study, three long time coating experiments were performed to test the described hypotheses. The inter-tablet uniformity was derived gravimetrically, while the intra-tablet data was derived using micro-computed tomography and confocal chromatic thickness determination. Towards the end of the coating experiments, a non-zero plateau of inter-tablet uniformity was reached. Furthermore, the data showed non-random deviations from the hypothesized one-over-square-root-of-time-model. The data for intra-tablet uniformity showed a non-linear decrease as well, but did not allow falsification of either hypothesis. It was additionally found that the cap-to-band ratio was below one at short coating times and increased to values above 1 during the process, which implies that existing declarations from literature might be limited to certain process durations.

Comparison of video analysis and simulations of a drum coating process

Tablet coating is a common unit operation in the pharmaceutical industry. To improve currently established processes, it is important to understand the influence of the process parameters on the coating quality. One of the critical parameters is the tablet velocity. In this work, numerical results are compared to results obtained experimentally. Tablet movement in the drums was simulated using the Discrete Element Method (DEM). The simulation parameters were adapted to fit the simulation to the experimental data. A comparison of the experimental and simulation results showed that the simulation correctly represents the real tablet velocity. A change in the velocity over time and its dependence on the rotation rates and the baffle position in the simulation were similar to the experimental results. In summary, simulations can improve the understanding of tablet coating processes and will thus provide insights into the underlying process mechanics, which cannot be obtained via ordinary experiments.

A comparative study between conventional pan coater and quasi-continuous small batch coater on the stability of tablets containing acetylsalicylic acid

The Supercell coater was developed as an in-line small batch tablet coater which uses air-fluidization for tablet coating. Coating time is very much reduced, with improved heat and mass transfer. It was hypothesized that the quasi-continuous Supercell coating process was more suitable for the aqueous coating of tablets containing moisture-sensitive drugs. Acetylsalicylic acid (ASA) was used as the model drug in this study. The extent of ASA degradation in Supercell coating was compared against that of tablets coated using the conventional pan coater. Less than 0.3% of ASA was degraded at the end of the coating process using either coater. The extent of ASA degradation was found to be more pronounced during storage. The Supercell coated tablets exhibited comparable or smaller percentage of ASA degradation than the pan coated tablets at the end of a storage period of 6 months under accelerated stability conditions (40°C/75% RH) and 3 years under ambient conditions (25°C/50% RH). The extent and rate of ASA degradation during storage were dependent on the processing conditions employed during Supercell coating. Increase in temperature generally led to a reduction in ASA degradation, while increase in spray rate and coating level caused more degradation. Greater extent of ASA degradation was observed on the surface of pan coated tablets compared with Supercell coated tablets due to greater moisture contact and the slower and wetter coating process. Changes to the processing conditions also influenced the residual moisture content (0.55-2.86%) of the tablets. However, no direct correlation between the residual moisture content of the tablets after coating and the extent of ASA degradation during storage was found.

Optimization of the inter-tablet coating uniformity for an active coating process at lab and pilot scale

The objective of this study was to enhance the inter-tablet coating uniformity in an active coating process at lab and pilot scale by statistical design of experiments. The API candesartan cilexetil was applied onto gastrointestinal therapeutic systems containing the API nifedipine to obtain fixed dose combinations of these two drugs with different release profiles. At lab scale, the parameters pan load, pan speed, spray rate and number of spray nozzles were examined. At pilot scale, the parameters pan load, pan speed, spray rate, spray time, and spray pressure were investigated. A low spray rate and a high pan speed improved the coating uniformity at both scales. The number of spray nozzles was identified as the most influential variable at lab scale. With four spray nozzles, the highest CV value was equal to 6.4%, compared to 13.4% obtained with two spray nozzles. The lowest CV of 4.5% obtained with two spray nozzles was further reduced to 2.3% when using four spray nozzles. At pilot scale, CV values between 2.7% and 11.1% were achieved. Since the test of uniformity of dosage units accepts CV values of up to 6.25%, this active coating process is well suited to comply with the pharmacopoeial requirements.